docs: def find_by_id(cls, id):
          """
          Finds a single document by its ID. Throws a
          NotFoundException if the document does not exist (the
          assumption being if you've got an id you should be
          pretty certain the thing exists)
          """
          obj = cls.find_one(cls._id_spec(id))
          if not obj:
              raise NotFoundException(cls.collection, id)
          return obj